Having a custom domain for email also makes it a lot easier to move email provider in the future if you want/need to since your addresses can stay the same as long as the new provider supports catch-all for all the aliases.

You could try Proton’s other domains first, because they might not all be blocked. Another option is to use a different company’s mail aliasing/forwarding or for these rare cases.

I do this as well. Bought one from njal.la for $15/yr